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Microwave Oscillator

Microwave Oscillator. By Professor Syed Idris Syed Hassan Sch of Elect. & Electron Eng Engineering Campus USM Nibong Tebal 14300 SPS Penang. One-port negative Oscillator using IMPATT or Gunn diodes. Negative resistance device is usually a biased diode. Oscillation

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Microwave Oscillator

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  1. Microwave Oscillator By Professor Syed Idris Syed Hassan Sch of Elect. & Electron Eng Engineering Campus USM Nibong Tebal 14300 SPS Penang

  2. One-port negative Oscillator using IMPATT or Gunn diodes Negative resistance device is usually a biased diode. Oscillation occurred whence ZL= -Zinwhich implies

  3. Stability of oscillation Oscillation takes place when the circuit first unstable, i.e Rin +RL < 0 . Rin depends on current and frequency. Any transient or noise will excite or cause oscillation . The oscillation will become stable when Rin +RL=0 and Xin +XL=0. The stable frequency is fo. Let’s ZT(I,s)= Zin(I,s)+ZL(s) Where I current and s=jw is a complex frequency. Then for a small change in current dI and in frequency ds, the Taylor’s series for ZT(I,s) is

  4. Continue (stability) Use the fact that Where ds=da+jdw Therefore If the transient caused by dI and ds to decay we must have da < 0 when dI>0 so that Or subst ZT=RT+jXT

  5. Continue ( stability) For passive load By substituting ZT=Zin + ZL, the stability equation reduces to Where Zin = Rin + j Xin ZL =RL + jXL

  6. Matching diode oscillator Eg. A negative -resistive diode having Gin=1.25 /40o (Zo=50ohm) at its desired operating point , for 6 GHz . Design a load matching network for one-port of 50 ohm load oscillator. By plotting ZL in Smith chart then match to 50 ohm as usual. The 50W

  7. FET oscillator • Choose high degree of unstable device. Typically, common • source or common gate are used.Often positive feedback to • enhance instability. • Draw output stable circle and choose GT for large negative • resistance (I.e Zin). Then take ZL to match Zin. Choose RL such that RL+Rin < 0, otherwise oscillation will cease.

  8. Design Usually we have to choose For resonation And For steady -state and where We can proved that

  9. FET common gate Design 4GHz oscillator using common gate FET configuration with 5nH inductor to increase instability. Output port is 50W. S- parameter for FET with common source configuration are : (Zo=50W) S11= 0.72/-116o, S21=2.6/76o, S12=0.03/57o,S22=0.73/-54o. 5nH

  10. continue First we have to convert from common source S-parameter to common gate with series inductor S-parameter. This is usually done using CAD. The new S-parameter is given by S11’= 2.18/-35o, S21’=2.75/96o, S12’=1.26/18o,S22’=0.52/155o. Thus the output stability circle parameters are given as where

  11. To determine GT Since S’11>1, thus the stable region is inside the shaded circle. GT can be choose anywhere in the Smith chart but the main objective Gin should be larger than 1. Let say we choose GT=0.59/-104. Then calculate Gin, thus Or Zin= -84 - j1.9 W Then

  12. Using a transmission line to match a resistive load, thus we have a length of 0.241 l and a load of 89.5 W. Using Rin/3 should ensure enough instability for the startup of oscillator. It is easier to implement ZL =90 ohm . The steady -state oscillation frequency will differ from 4Ghz due to the nonlinearity of the transistor 0.346l For GT matching, we can use open-stub to match 50 ohm. Plot GT and then determine the YT. Moving towards load until meet the crossing point between SWR circle and the unity circle. That the distant between transistor and the stub. Obtain the susceptance and distance towards open circuit. Towards generator GL 0.319l GT

  13. Dielectric resonator Equivalent series impedance Where N =coupling factor/turn ratio Q=R/woL (unloaded resonator) Ratio of unloaded to external Q is given by where RL=2Zo for loaded resistance = Zo for l/4 transmission line

  14. Continue (Dielectric resonator) Reflection coefficient looking on terminated microstrip feedline towards resonator is given by or Q can be determined by simple measurement of reflection coefficient

  15. Dielectric resonator oscillator Series feedback Parallel feedback

  16. Example (dielectric resonator osc.) Design 2.4GHz dielectric resonator oscillator using series feedback with bipolar transistor having S-parameters (Zo=50ohm); S11= 1.8 / 130o , S12= 0.4 / 45o , S21= 3.8 /36o, S22= 0.7 / -63o. Determine the required coupling coefficient for dielectric resonator and matching. Solution Circuit layout

  17. continue • Procedures • Plot the stability circles 2. Choose a point Gin Inside the instability area

  18. continue Calculate the Gout and Gin = GL using this formula We obtain Gout = 10.7/132o. This corresponding to Then

  19. Continue (output matching) X So we have d1=0.034l l1=0.193 l Or d1=0.429l l1=0.307l

  20. Network at resonator Resonator should be placed at zero or 180o of phase from the transistor. So we have either 0.181 l (zero phase) or 0.431 l (180o phase) d2= 0.181 l Or = 0.431 l

  21. Noise in oscillator • Amplitude noise • Phase noise • Flicker noise Phase noise-may be due to variation of device capacitance with variation of voltage.This is usually happened in amplifier.Amplitude noise may be converted to phase noise if the amplifier is present. Noises cause frequency instability in oscillator.

  22. Noise to Carrier Ratio (NCR) Parallel impedances for Rp , Lp , and Cp can be written as where and

  23. NCR Limit (cont) The transfer function of the oscillator is given by Then substitute for Zp , we have

  24. NCR (cont) At oscillation Where fo=oscillation frequency And the gain condition (Barkhausen) for oscillation is gmRp=1 Thus, any changes will result #%

  25. NCR (cont) In the oscillator model, the noise source is Rp .The noise current produced is where k=Boltzman const , T = absolute temp. B= bandwidth Since gm= 1/Rp and Iout= gm* Vin , the noise current can be transferred to input and hence Vin can be written as **%%

  26. NCR (cont) Thus the Vout, can be obtained by substituting and squaring #% and **%% . We have Taking B= 1 Hz and carrier voltage ,Vcarrier-rms And the carrier power is given by The noise to carrier ratio for SSB in Hz is given by Where fm =offset frequency from carrier

  27. NCR (cont) For phase noise Note: This ratio is half of the total noise since half will be converted to AM noise and half left for phase noise.

  28. Example Calculate the phase noise to carrier ratio of an oscillator of 10MHz with Q=100. Assume the inductor is 2 mH and the peak voltage across it is 10V. Let the noise figure is 10dB.

  29. Flicker noise ( 1/f noise) As in previous example fm NCR 50kHz 170dB/Hz 30kHz 168.5dB/Hz 10kHz 159dB/Hz

  30. Design for low 1/f noise • Design procedures:- • Choose high Q-factor of the resonator • Choose low 1/f noise active components (e.g Bipolar transistor) • Choose transistor with the lowest possibility of fT . For good rule of thumb fT< 2 x fosz . • Low current best 1/f performance. Note that fT drops with low current. (FET) (HBT) (BJT) Maximum oscillation frequency • For high Q-factor choose parts that have low losses: • Resonator • Series resistance of capacitors • Series resistance of tuning diode • PCB.

  31. Measure phase noise from VNA (for checking) • Verify power input signal no higher than 10dBm • Reduce input attenuation to minimum (0 dB) • Determine the carrier power at large video and resolution bandwidth at appropriate span (3MHz RBW, 1MHz VBW,50MHz span. • Set span for single sideband ( desired offset frequency) • Reduce VBW to 10 Hz, RBW to 1 kHz. • Set marker to the carrier. Select marker to show the frequency offset. • Move the marker along the SSB phase noise curve and take reading. MAX HOLD for maximum phase noise power( let the spectrum settle for 5 minutes ) • Note that cable insertion loss should also be determined

  32. Measure phase noise from VCO

  33. Reducing Phase Noise in Oscillators 1. Maximize the Qu of the resonator. 2. Maximize reactive energy by means of a high RF voltage across the resonator. Use a low LC ratio. 3. Avoid device saturation and try to use anti parallel (back to back) tuning diodes. 4. Choose your active device with the lowest NF (noise figure). 5. Choose a device with low flicker noise, this can be reduced by RF feedback. A bipolar transistor with an unby-passed emitter resistor of 10 to 30 ohms can improve flicker noise by as much as 40 dB. - see emitter degeneration 6. The output circuits should

  34. YIG oscillator Condition for oscillation S11’>1 and S22’>1

  35. YIG equivalent circuit fo=resonance frequency=nHo where V= volume of YIG sphere k=1/d1=coupling factor and d1 is the loop diameter wm= 2pfm=2pn (4p Ms) Ho= dc magnetic filed n= gyro magnetic ratio ( 28 GHz/Tesla) DH= resonance line width L1= self inductance of the loop 4pMs= saturation magnetism

  36. Hartley Oscillator

  37. Colpitts Oscillator

  38. Effects of ambient changes on stability in oscillators A frequency change of a few tens of hertz back and forth over a couple of minutes would mean nothing to an entertainment receiver designed for the FM Radio band. Such a drift in an otherwise contest grade receiver designed to receive CW (morse code) would be intolerable. It's a question of relativity.

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